Ventilation Perfusion Abnormalities
Ventilation of unperfused alveoli leads to increase in dead space. Perfusion of unventilated alveoli results in the addition of unoxygenated blood to pulmonary venous blood. Taking effective minute ventilation to be 4 litres and pulmonary blood flow to be 5 litres, the normal ventilation perfusion ration is 0.8. Several conditions lead to imbalance in ventilation.
1. Normal variations
The upper parts of the lungs receive less blood than the bases, the hydrostatic pressure of the blood being higher at the base. Though the perfusion is unequal is the different portions of the lung, ventilation is more or les uniform with only minor differences.
2. Obstruction to pulmonary blood flow
Pulmonary blood flow is obstructed in pulmonary embolism, vascular changes due to chronic inflammation, destructive lesions of the lung, pulmonary hypertension and vasoconstriction due to hypoxia. Since carbondioxide is 20 times more easily diffusible than oxygen, the ventilation perfusion imbalances lead to hypoxia, but not to hypercapnia, except in the advanced cases.
Gas exchange by diffusion across the alveolar membrane
The alveolar membrane is 0.2 to 0.7 micrometer and it consists of a single layer of cells lining the alveoli, a thin basement membrane and the endothelial cells of the capillary. The alveolar capillaries contain mixed venous blood with carbondioxide and low oxygen tensions. Oxygen passes into the capillary and carbondioxide passes into the alveoli within a few milliseconds, when the alveolar membrane is thickened, gas exchanged is impaired. Diffusion of oxygen is affected early and, therefore, hypoxemia occurs first. Retention of Carbondioxide occurs only when the lesion is advanced. Several factors such as the tructures of the abnormalities affect diffusion capacity to a great extent. Hence the term "transfer factor" is used instead of diffusion capacity.
Blood Gases
Both oxygen and carbondioxide are carried by blood. Diffusion across the alveolar membrane depends upon the partial pressure of these gases on either side, of the diffusing capacity of these gases. Since carbondioxide is much more readily diffusible than oxygen, the level of carbondioxide in blood closely follows the partial pressure of carbondioxide in alveolar air. The pattern of oxygen dissociation curve of hemoglobin is such that partial pressure of oxygen in arterial blood does not fall significantly even when the partial pressure of oxygen in the alveoli falls from 100millimeters Mercury to 80 millimeter Mercury. But when the alveolar Oxygen falls below 80 millimeters Mercury, arterial Oxygen falls steeply. The arterial Oxygen concentration does not closely follow the alveolar oxygen concentration due to this phenomenon. Oxygen is carried by blood mainly in combination with hemoglobin (1.34ml/g of Hb) and a small quantity as the dissolved form (0.003ml/100 ml blood/mm Hg of oxygen tension). The oxygen content in blood can be expressed either as the percentage saturation or the partial pressure. Arterial carbondioxide level is expressed in terms of its partial pressure. Alveolar gas concentrations are expressed in terms of their partial pressures.
Normal blood gas values
Arterial oxygen concentration (95-98)%; Partial pressure of arterial oxygen (80-100 mm Hg [11-14KPa]); Partial pressure of arterial carbondioxide (35-45 mm Hg [4.5-6.0KPa]).
Pulmonary mechanics- Work of breathing:
The total work involved in moving the thoracic cage, expanding the lungs and moving the gases in and out is known as the work of breathing.
Pulmonary compliance:
The elastic property of the lung is expressed in terms of pulmonary compliance. It is the distensibility of the lung per unit change in intrapleural pressure. Normal pulmonary compliance is about 0.2L per cm of water. In conditions like emphysema where there is loss of elastic tissue, lung is more distensible and in conditions like pulmonary fibrosis and pulmonary edema compliance is diminished.
Airway resistance:
About 90% of resistance to flow of air is contributed by the larger air passages and 10% by smaller airways. Airway resistance is expressed as H2O/liter/second. Resistance to air flow offered by the air passages depends upon several factors like the caliber of the passage, driving pressure, rate of flow, type of flow (laminar or turbulent), density of the gas, and its viscosity. Airway resistance is calculated from values for atmospheric and alveolar pressures and the rate of air flow.
Airway resistance = atmospheric pressure - alveolar pressure
Rate of flow
In general, airway resistance is measured at a flow rate of 0.5 litres/sec. In normal, during quiet breathing the airflow resistance varies between 1.5 to 3cm water/litre sec. It reaches, high values (above 10cm of water/litres/sec) in obstructive airway disease.
1. The lung volumes: Several parameters are used to determine the ventilatory capacity of the lung.
- Tidal Volume (VT) is the volume of gas inspired or expired during each respiratory cycle. Normally it is 0.5 liters.
- Inspiratory reserve volume (IRV) is the maximal volume of gas that can be inspired from the end of tidal inspiration. Normal value is 2 liters..
- Expiratory reserve volume (ERV) is the maximal volume of gas that can be expired from the end of tidal expiration. Normal value is 1.3 liters.
- Residual Volume (RV) is the volume of gas still remaining in the lungs after maximal expiration. Normal value is 1.6 liters.
2. The Lung Capacities
Total lung capacity (TLC) is the volume of gas contained in the lung at the end of maximal inspiration. Normal value is 5.4 liters.
Vital Capacity (VC) is the maximal volume of gas that can be expelled from the lung by forceful effort after maximal inspiration. In health, vital capacity is influenced by factors such as age, sex, position, body frame and state of physical conditioning. Average normal value is 3.8 liters.
Inspiratory Capacity (IC) is the maximal volume of gas that can be inspired from the resting expiratory level. Normal value is 2.5 liters.
Functional residual capacity (FRC) is the volume of gas remaining in the lung at the end of tidal expiration. Normal value is 2.9 liters.
Forced expiratory volume in one second (FEV1) [timed vital capacity]: The volume of air expelled in the first one second of a forcible expiration following a full inspiration is called forced expiratory volume in one second (FEV1). Normally FEV1 is above 75% of the total vital capacity, FEV2 is above 85%, and FEV3 is above 95%. Airways obstruction is indicated by FEV1 below 70% of normal.
Forced expiratory time (FET) is the total time taken for completing a forced expiration. Normally it is less than 4s.
Peak expiratory flow rate (PEFR) is the maximum rate than can be sustained during the first 10 millisecs, of a sudden forced expiration after a full inspiration. The PEFR depends up on the height and surface area of the individual. Nomogras are available for reference.
Maximal expiratory flow rate (MEFR): This is the flow rate at a specified portion of a forced expiration after a maximal inspiration, e.g MEFR 300-1300 denoted flow rates for 1 liter of expired gas after the first 300 ml has been breathed out.
Maximal inspiratory flow rate (MIFR) This is the flow rate at a specified portion of a forced inspiration.
Maximal mid-expiratory flow rate (MMFR): This is the velocity of air expressed as liters per second during the middle third of the total expired volume. It is also denoted as forced expiratory flow (FEF 25-75%). In normal the values vary with age and height of the individuals. Average values lie between 1.5 and 5.5 liters/sec in men. Determination of MMFR helps in detecting borderline cases of airways obstruction.
Maximal voluntary ventilation (MVV), or maximal breathing capacity is the total volume of air breathes by a subject using maximum effort over a period of 1 min.
Air velocity index (AVI): The ratio of the percentage of predicted MVV to the percentage of the predicted vital capacity is called air velocity index. The normal range is from 0.8 to 1.2.
Diffusing capacity: This is the volume of a gas transported across the alveolo-capillary membrane in 1 min for one unit of pressure gradient. It is expressed as ml/min/mm of Hg difference in partial pressure.
Closing capacity: During inspiration the air centers different portions of the lung in a definite order.
The upper portions fill first and then the middle and lower parts in order. During expiration air escapes in the reverse order, the basal portions emptying first and the apical regions being the last.
As a result, the smaller airways at the bases start to close even while air from apices is escaping.
The volume of air contained in the lungs at the point where the airways first start to close is called closing capacity.
Closing Volume (CV): The difference between the closing capacity and the residual volume is termed closing volume. This is often expressed as a percentage of the vital capacity. In normal subjects below 40 years it is less than 20%. The CV increases with age. In many cases increase in closing volume may be the only detectable abnormality in impending airway obstructions.
Assessment of Pulmonary function:
Different aspects of respiratory functions can be subjected to investigational study, these include:
1. Gas transport down the airways,
2. Gas mixing within alveoli
3. Gas transfer across the alveolocapillary membrane, and
4. Lung perfusion.
Entry of air down the airways and its return can be measured using static and dynamic spirometry. Body plethysmography is employed to measure lung compliance and airways resistance. FEV1 and VC which are the most important parameters to assess the ventilatory capacity, are estimated by spirometry.
Peak expiratory flow rate:
It is measured using Wright's peak flow meter. This is an easy and convenient method to assess airways obstruction. Other methods to assess PEFR employ the peak flow gauge and the De Bono Whistle, Distribution of inspired air in different parts of the lungs is studied by single breath oxygen test.
Tests for studying gas mixing within the alveoli
These measurements demand rapid analysis of expired air. The measurement of alveolar and arterial carbon dioxide tension is the method employed for this purpose. Estimation of transfer factor is conducted by using carbon monoxide which has a very high diffusing capacity.
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Ventilation of unperfused alveoli leads to increase in dead space. Perfusion of unventilated alveoli results in the addition of unoxygenated blood to pulmonary venous blood. Taking effective minute ventilation to be 4 litres and pulmonary blood flow to be 5 litres, the normal ventilation perfusion ration is 0.8. Several conditions lead to imbalance in ventilation.
1. Normal variations
The upper parts of the lungs receive less blood than the bases, the hydrostatic pressure of the blood being higher at the base. Though the perfusion is unequal is the different portions of the lung, ventilation is more or les uniform with only minor differences.
2. Obstruction to pulmonary blood flow
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